62 CHARLES E. STOCKARD 



the liver are not blood forming cells, the only blood formed 

 within the liver arises from existing blood 'cells which are carried 

 there in the circulating current. 



In this manner the multiplication of blood cells is shifted from 

 place to place and becomes more and more localized until ulti- 

 mately in higher animals the red bone marrow is the only body 

 tissue in which these open spaces furnishing the required environ- 

 ment exist. It is, therefore, the only body tissue in which 

 erythroblasts live and continue to multiply and give rise to 

 the entire stock of red blood corpuscles which circulate through- 

 out the body. 



Blood cells always multiply in the unlined spaces but normally 

 never multiply to any extent within closed vessels. It might 

 be possible that certain abnormal growth tendencies on the part 

 of the endothelium of the vessels and sinusoids in the bone 

 marrow might cause an inclusion or vascularization of the 

 spaces in which blood cells multiply and this growth might indi- 

 rectly result in the cessation of the production of red blood cor- 

 puscles. This might be experimentally tested should some 

 method be devised by which the growth of vascular endothelium 

 could be so stimulated as to close the spaces of the bone marrow. 



8. Question of haematopoetic organs 



In the fish embryo, the haematopoetic function of the liver 

 is not of great importance. Yet in the liver of normal individuals 

 numbers of blood cells are always found and numerous dividing 

 blood cells are present. In the non-circulating embryos the blood 

 is unable to reach the liver and in such cases there are no blood 

 cells of any type to be seen in this organ. 



Figure 49 represents a section through the gall bladder, bile 

 duct and body of the liver in a sixteen-day old embryo. In 



Fig. 49 A section through the liver of a sixteen-day-old embryo, without a 

 circulation; Embryo 413, 1913. The gall-bladder, GB, and bile duct are seen 

 connected with the intestine, Int; the liver, L, is a compact mass containing 

 neither vessels nor any type of blood cell. A, the well developed dorsal aorta 

 is lined by endothelium but its lumen is completely empty except for a slight 

 coagulum near the center; icm, if followed posteriorly leads into the remains 

 of the intermediate cell mass; WD.. nephric duct; Nch, notocord. 



